Early Life and Career
Election as First Citizen
Reign as First Citizen
Relationship with Carausius
Even though Tetricus had honored the letter of his promise to designate Carausius his successor, he had no intention of keeping the spirit of his promise. Throughout his rule, Tetricus marginalized Carausius and Carausius' allies. Important civil and military posts were reserved for Tetricus' partisans, whereas Carausius and his allies were relegated to positions where they could not gain meaningful leadership experience. Tetricus hoped to make it so that when Carausius would become First Citizen, he would be dependent on people more qualified for the office than himself and who had been loyal allies of Tetricus.
Relationship with the Senate
Tetricus had won the support of the Postumian faction during his campaign for the emperorship; and after his election, he continued to sympathize with the Postumians. As he promised, he restrained himself from casually stuffing the Senate with his allies. Doing so would have alienated the Postumians — who were the majority of his supporters — and angered those who had supported Laelianus and Carausius.
Despite Tetricus' self-restraint and respect for the institution of the Senate, his relationship with the Senate was often contentious. Tetricus had only been elected First Citizen by a plurality vote: the majority of the senators had opposed him, but could not unite behind either Laelianus or Carausius. These senators would sometimes challenge Tetricus. The group that had supported Carausius was especially hostile to Tetricus, because of how blatantly Tetricus marginalized Carausius.
The Amasigan War
Roman authority had steadily weakened in the Mauritanian provinces during the third century. This had enabled Berber tribes to begin invading the area. Sometimes the Berbers raided cities and towns, but sometimes they occupied them. Messalla made no attempt to engage the invaders, as he had other priorities. Tetricus, on the other hand, was interested in coming to Mauritania's aid. Like his predecessors, Tetricus considered it important for the Gallic Empire to annex Mauritania Tingitana, and he saw the Berber invasions as an opportunity to gain influence in the area. (Like Victorinus, he wished for the Gallic Empire to annex Mauritania Tingitana only after it was clear that the people of the province would support annexation.)
In April 295, Tetricus sent a force of twelve thousand to the area just south of the province, and the units arrived at their destination a month later. During the summer, the Gallic forces liberated several Roman outposts that the Berbers had occupied and intercepted several bands of marauders. It was not until September that the Gallic forces began an offensive into the Atlas Mountains, which was Berber territory. The campaign proved difficult enough that an extra six thousand men had to be deployed from Baetica and Lusitania in November 295. Ultimately, the Gallic campaign proved to be a success: the Gauls had ventured deep into Berber territory, where they plundered and burned many villages. The offensive ended in April 296.
While the war was still in progress, Tetricus had discussed what to do with the Berber lands with the Senate. Tetricus believed it would be a good idea to permanently occupy some of the territory south of the Roman border. Many senators agreed with this, but there was no consensus as to exactly which areas should be annexed. The Senate finally reached a decision a month after the war ended: the area between the Bou Regreg River, Oum Er-Rbia River, Atlantic Ocean, and Atlas Mountains would be incorporated into the Gallic Empire as the province of Amasiga (derived from the Berbers' name for themselves), with Anfa (OTL Casablanca) as the province's capital. Some senators also pushed for the annexation of a seemingly insignificant section of Mauritania Tingitana, but the Senate rejected that motion.
The Amasgian War, as the war came to be known, accomplished two things. It put Mauritania Tingitana in Gaul's debt and opened the door for Gallic influence: the people of the province would remember that Tetricus had ordered the campaign to expel the invading Berbers at a time when Messalla and Maximian had other priorities. At the same time, it gave the Gallic Empire a toehold in Africa immediately.
In general, Tetricus continued the policies of the preceding emperors toward the Germanic tribes. He worked to maintain and expand the Gallic Empire's sphere of influence east of the Rhine by building trade and diplomatic relations with other tribes. During the 290s, he continued Victorinus' efforts to forge ties with the Franks and Frisians. The Pannonian and Norican states also continued to forge ties with their neighbors.
Not all the Germanic tribes were interested in friendly relations with Gaul, however. Several times during the 290s, the Saxons raided both the Franks and the Gallic Empire. This led to two punitive campaigns against the Saxons in the late 290s. Similarly, the Asdingi branch of the Vandals raided Quadium once in 292 and again in 296. Problems were also emerging in the area just north of the Danube by the time Tetricus became First Citizen.
The First Burgundian-Aquilonian War
During the late 280s, the Burgundians and branches of the Hermanduri and Chatti migrated into the area north of the Danube, the former homeland of the Alamannians, Marcomannians, and Suevians. At the same time, not all of the Alamannians, Suevians and Marcomannians had chosen to migrate to Pannonia or Noricum; and the branches of those tribes had come to be known collectively as the Aquilonii (Northerners). By the time Tetricus had become First Citizen, a clearly tense relationship had emerged between the Aquilonii and the newcomers. The old tribes saw the arrival of the new tribes as a threat to their independence. From the point of view of the arriving tribes, much of the land immediately north of the Danube was uninhabited, so it was theirs to settle. Tetricus desired friendly relations between all the groups in the area, but he knew that if the situation north of the Danube degenerated into hostilities between the old and new tribes, he would likely be forced to take sides. As for the states of Alamannia, Marcomannia and Suevia, they were strongly interested in the well-being of the Aquilonii.
Several times between 295 and 297, delegates from Gaul, the Pannonian and Norican states, each Aquilonian group, and each of the new tribes met in Rhaetia to try to formulate an agreement regarding the area north of the Danube that would satisfy the concerns of every party. The conferences failed: no plan proposed satisfied every party involved. Then during the latter half of 297, the relationship between the Aquilonians and the Burgundians degenerated into violence: first riots broke out in villages with mixed populations, and then Burgundians began attacking villages inhabited mainly by Aquilonians. This prompted the Alamannian and Marcomannian governments to send military forces to protect the Aquilonians.
The Alamannian and Marcomannian forces arrived in Aquilonian territory by January 298. By March, it was clear that the Burgundians had an advantage over the Alamannian-Marcomannian-Aquilonian force. Tetricus decided that it was necessary to aid the Aquilonians, so he sent twenty thousand soldiers to attack the Burgundians. The Gallic force was large enough to drive the Burgundians out of the predominantly Aquilonian areas. In June 298, the Burgundians informed the generals leading the Gallic legions of their intent to surrender. The Gallic generals then informed the Burgundians of the terms of peace: Aquilonian-inhabited areas were to be independent of Burgundian control, and the Burgundians were not to settle in or near any village or town inhabited by Aquilonians.
This arrangement would ultimately prove to be unsustainable. The Burgundians had been devastated by the Gallic army, but they would recover. As for the Aquilonians, their population was simply too small for them to effectively control the territory they were guaranteed. During the war, some Aquilonians migrated to Pannonia and Noricum, and this trend continued after the war. Also, the Aquilonians never united as a single political entity, something that would limit their ability to effectively resist future aggression.
By 304, Carausius had decided that the only way he could ever hope to rule without being a puppet of allies of Tetricus was by having Tetricus assassinated and seizing the throne by force. During the spring and summer of 304, Carausius conspired with a longtime friend of his named Sextus Aurelius Allectus to have Tetricus killed.
The assassination of Tetricus took place on the morning of September 8, 304. On that day, Tetricus was on his way to the Senate building. Five archers who had been trained by Allectus were hiding at different points near the entrance. As Tetricus began to walk up the steps that led to the door, all five of the assassins shot arrows at him. Each assassin had dipped his arrows in poison. Three of the arrows hit Tetricus, and he fell to the ground. Each of the assassins shot a second arrow; and one of those arrows hit him, and three hit his bodyguards. Thereafter, all five of the assassins fled the scene. Tetricus and the two bodyguards of his who were shot died within minutes.
The assassination of Tetricus was the start of a chain of events that would lead to the First Gallic Civil War. Soon after Tetricus died, Carausius arrived in Colonia Agrippina and assumed the emperorship. Suspicion that Carausius was involved in the assassination of Tetricus immediately arose. Also, the Postumians feared that Carausius would either purge them from the Senate and the military or create new Senate seats for his partisans to dilute their influence. On February 16, an overnight Senate meeting was held, during which the Senate voted to depose Carausius and appoint Aurelius Arpagius First Citizen. Carausius and the dissenting senators refused to recognize the vote on procedural grounds. Army units began choosing sides. On February 24, 305, the civil war began when troops loyal to Carausius unsuccessfully attempted to arrest the senators who had voted to depose him.